Double mechanical seals are commonly placed at the interface between a process pump and the rotatable shaft which drives the pump. The double mechanical seal is there to avoid loss of fluid from the pump in the area where the rotatable shaft is inserted into the process pump. The rotatable shaft is usually powered by a motor.
Most double mechanical seals have a cavity defined by the sealing faces, the gland housing and the rotatable shaft through which a barrier fluid is circulated to support the correct running of both sets of faces by cooling the seal. The barrier fluid is stored in a header tank and circulated to the seal by means of inlet and outlet pipes. At present, there are two main systems for circulating the barrier fluid. The first makes use of a thermosyphon and the second a separate circulating pump.
The thermosyphon system allows for heat to be removed from the seal faces by the circulation of water. As the water is heated, it expands and thus becomes less dense than the incoming, cool water. Placing the water outlet from the seal cavity above the inlet ensures that the heated water is ducted out of the seal cavity and escapes back to the header tank. As a result, cool water is drawn in through the water inlet.
It is sometimes preferable to use oil rather than water as the barrier fluid, for example where the product being sealed is incompatible with water. Because oil does not expand sufficiently to thermosyphon when it is heated, it has to be pumped around the system. Barrier fluid also has to be pumped where large amounts of heat have to be removed from the seal, for example where the equipment is being used with explosive chemicals in which the build-up of heat could be extremely hazardous, or where a pressure differential is required to ensure that the barrier fluid is on seal faces and not the product.
In these circumstances, a second motor has been used to drive the barrier fluid pump. However, the use of a second motor can be problematical in areas where there are explosive chemicals, because the propensity of the motors and their electrical connections to cause electrical sparks can be a fire hazard. Furthermore, the use of additional pressurising pumps has historically been extremely expensive, because they are used in hazardous chemical environments and are therefore required to meet stringent safety requirements. The header tank itself, used in the pressurised system, must also be manufactured to ASME VIII standard.
It has been proposed to avoid the necessity of using a second pump and motor by incorporating fins onto that part of the seal which is attached to the rotating shaft or otherwise modifying the shape of the seal cavity to as to allow flow to be induced by the rotation of the shaft. However, it has been found that such designs are rather less effective than might have been hoped and may not perform well enough for critical hazardous chemical systems in that, whilst they create a limited flow, they do not generate enough positive pressure to effect a pressure differential across the seal faces.